Buckminsterfullerene (C60 fullerene) is a stable carbon molecule in the form of sphere, and composed of 60 carbons. In the C60 fullerene, all carbon has same conditions because it forms icosahedral structure, this is confirmed by the single peak of 13C-NMR. Since C70 and C80 fullerenes as well as C60 fullerene can be synthesized in the limited amount, C60 is mainly synthesized and studied.
Most fullerene molecules are in the form of almost sphere symmetry and are nonpolar, so that they do not dissolve in polar solvent such as water or alcohol, but dissolve in nonpolar solvent such as benzene or toluene. Also, fullerene molecules are very sensitive to light. Due to such photosensitive property of the fullerene which turns to excited state by light, fullerene can easily be radical or photo-sensitizers. The electro-chemical properties of fullerene are very useful and 6 reversible oxidation-reduction reactions are available in solution. Also, the industrial availabilities and practical abilities of fullerene have recently gained increasing attention due to unique structure and hard property, and superconductivity of fullerene mixture with alkyl metals.
As mentioned above, fullerene is very sensitive to light. To be specific, the light absorption is particularly high at ultraviolet regions (213, 257, 329 nm). Yet, fluorescent property of Fullerene, which can be represented as quantum efficiency (QE) of fluorescence, has been reported relatively low.
The quantum efficiency of fluorescence indicates the number of fluorescence photons emitted per absorbed photons. QE of fullerene is approximately 1˜2×10−4 at atmospheric temperature. Due to such low efficiency of fluorescence, there are few cases that utilize fluorescence of fullerene. Further, despite the optical, electrical properties, fullerene has difficulties for its applications which are, mainly, low solubility to organic solvents or low self agglomeration. In particular, fullerene is so sensitive to the surrounding environment that its physical and chemical properties are easily changeable. Therefore, various regulating methods of fullerene have been developed.
The above methods include doping fullerene in host matrix, keeping fullerene in porous inorganic materials, or using sol-gel materials. The sol-gel process is particularly useful to synthesize fullerene nano-composite with low reaction temperature and relatively easy chemical reactions. Also, the methods of fullerene-silica aerogel composite (Zhu et al. J. Phys. Chem. Solids 59, 819, 1998), incorporation of fullerene into porous VIP-5 Zeolite (Lamrabte et al. Chem. Phys. Lett. 295, 257, 1998), or sol-gel process of glass containing fullerene (Peng et al. J. Sol-Gel Sci. Tech, 22, 205, 2001) have been proposed. As explained above, although the fullerene-silica composite has been prepared to be utilized as optical materials based on the unique optical properties of fullerene, few has been developed to actual application. Meanwhile, the fullerene composites prepared by the above methods are obtained not in the form of nano-particles but as a bulk.
In recent, nano science has gained the spotlight, and study of synthesizing nanoparticles with fullerene has been conducted. Some examples include using fullerene derivatives to synthesize a fulllerene-silica hybride nano composite (Patwardhan et al. J. Inorganic and Organometallic polymers, 12, 49, 2002), and coating fullerene containing hydroxyl group with silica under the acidic condition (Whitsitt et al, Chem. Comm. 1042, 2003).
However, the fullerene-silica nanocomposites have irregular sizes and shapes, fullerene and silica are linked with separate linkers, and the fluorescence properties of the fullerene-silica nanocomposite have not been fully studied.
Therefore, to solve the above problems, there are needs for the synthesis of uniform fullerene-silica nanoparticles sized tens of nanometers, and studies an the optical properties of fullerene-silica nanoparticles, especially the fluorescence properties of the fullerene-silica nanoparticles.
The present inventors have been studying the preparation of fullerene-silica nanoparticles in uniform shape, and completed the present invention by discovering that by synthesizing fullerene-silica nanoparticles in the uniform shape with the method of reverse micro emulsion, in which the fullerene and silica are directly linked without requiring linkers, the synthesized nanoparticles exhibit strong fluorescence and thus can be used as a bioimaging agent or as a drug delivery carrier.